All my past invoices and customer details have disappeared from the app! Receipts still there. Please help!

Welcome to the Community, Gian. It's my pleasure to help you retrieve your customer information and invoices.

Before anything else, you may want to log in to the web version to verify if it is only happening in your QB app.

Refreshing the application will bring back customer-related data and transactions in the interim. It is the same as clearing the cache from your web browser. 

For iOS:

1. Go to the Help menu.
2. Choose Reset App Data.
3. Select OK.

For Android:

1. Proceed to the Home menu or tap the three dots in the right upper right corner.
2. Select the Settings tab. 
3. Choose the Refresh Data option.
4. Pick Yes.

Moreover, you can check this article to learn which QuickBooks Online mobile app features are supported on mobile devices: Compare mobile app features.

Feel free to drop by again if you have other QuickBooks-related concerns. We're available 24/7 to help you. Have a nice day, Gian!